1. Field of the Invention
The present invention relates to a plating process and, more particularly, to a method for providing a partial plating onto a relatively small article such as a lead pin capable of being used for electronic devices.
2. Description of the Related Art
Conventionally, a lead pin adapted to be incorporated in an electronic device, such as a semiconductor package, is plated over the entire surface of the lead pin. Various characteristics or performances are usually required for such a lead pin, so that it has been a general practice to provide a composite or multilayer plating onto the lead pin, which is formed by stacking various metal coatings one on top of the other. One of the typical conventional composite platings is a gold plating including a nickel base layer and a gold uppermost layer deposited on the nickel base layer.
The lead pin, the entire surface of which is coated with the gold plating including the nickel base layer, can be suitably used and incorporated in a semiconductor package named "a multichip module" in which plural semiconductor chips are mounted on a single substrate. In the multichip module, the lead pin is normally fixed at one end thereof to the surface of the module substrate by a gold/tin alloy solder (an Au/Sn solder).
The multichip module is adapted to be mounted on the surface of a circuit board through numbers of lead pins arranged in an array. When maintenance, such as chip replacement, is required for the multichip module, an operator usually performs the maintenance to a multichip module fully disengaged from the circuit board.
To facilitate such a disengagement of the module from the circuit board, each lead pin is generally connected at an end thereof, opposite to the fixed end on the substrate, to the circuit board through a tin/lead alloy solder (an Sn/Pb solder), the melting point of which is lower than that of the Au/Sn solder. In this structure, the lead pins are heated to melt only the Sn/Pb solder, which permits the module to be readily disengaged from the circuit board.
When the Sn/Pb solder for connecting each lead pin to the circuit board is molten, the molten Sn/Pb solder may flow on the gold plated surface of the lead pin to reach the Au/Sn solder for connecting the lead pin to the module substrate. Therefore, if one multichip module is repeatedly disengaged from and mounted to the circuit board during maintenance, the melting point of the Au/Sn solder may be lowered due to the mixing of a gold/tin alloy and a tin/lead alloy. When the melting point of the Au/Sn solder is lowered, not only the Sn/Pb solder but also the Au/Sn solder are molten during subsequent maintenance work, which may cause the lead pin to fall off from the module substrate.
To solve the problem of the falling-off of the lead pin, it may be beneficial to provide means for impeding the flow of the molten Sn/Pb solder to a gold plated portion defined between the solders on the opposed ends of the lead pin. Such means may be effectively structured by forming an annular exposed region having no gold uppermost layer, such as a nickel base-layer surface region or a lead-pin material surface region, over a predetermined length on the gold plated portion between the solders. In general, a nickel base-layer surface or a lead-pin material surface is more readily oxidized in comparison with the gold plated surface and is thereby provided with less wetability for solder, so that the flow of the molten Sn/Pb solder can be impeded at the annular surface region of the nickel base-layer or the lead-pin material.
Both the annular nickel base-layer surface region and the annular lead-pin material surface region may be formed by completing a gold plating process only in the opposed end regions of the lead pin, or by sanding the surface of the gold plated portion between the solders of the opposed ends to partially remove the uppermost gold layer after completion of the gold plating process. However, these processes are difficult to precisely perform on a minute lead pin, such as the lead pin of the multichip module, and also be unsatisfactory for mass production of the lead pin.